There are generally two types of cast irons which can be plastically deformed, those being malleable and ductile iron. Malleable cast irons are capable of being extended in all directions by hammering or rolling and typically contain about 0.8 to about 1.2 weight percent silicon and about 2.3 to about 2.8 weight percent carbon. Ductile cast irons are capable of being lengthened or flattened out, without losing continuity, when subjected to tensile stresses or rolling and typically contain about 2.2 to 3 weight percent silicon and about 3.4 to 3.8 weight percent carbon.
With either type of cast iron, most prior art practice has indicated that having carbon in predominantly graphite form is more desirable than having it in carbidic form. In typical graphite-containing cast irons, graphite precipitates and forms nodules upon cooling. When the alloy is further cooled to freezing, austenite forms around the graphite particles. The first austenite formed surrounding the graphite nodules will have a relatively high amount of silicon and will reject manganese.
Therefore, the manganese accumulates at the cell boundaries of the matrix and creates a non-uniform material with non-uniform physical properties. It has been known that elemental manganese may become as much as 10 times more concentrated at the cell boundaries than elsewhere in the matrix in typical graphite-containing cast irons. A non-uniform material with these high local concentrations of manganese are inherently weak in those areas after heat treatment, which may ultimately be the cause of premature failure due to breaking. In addition, graphites generally do not contribute to the strength of a cast iron, because they form a weak link to the cast iron matrix. Therefore, in prior art practice, the resulting cast iron products were not optimum in strength due to the higher volumes of graphite.
Examples of prior art cast irons and methods for making them are described in the following patents:
U.S. Pat. No. 2,749,238 to Millis, et al. discloses a method for producing a cast ferrous alloy containing at least about 50% iron, particularly at least about 87% iron, and carbon and silicon within the cast iron range, the carbon being in excess of that required to form the matrix being predominantly in the uncombined form, and containing a small but effective amount of magnesium to control the form of the uncombined carbon. The patent discloses that typical ferrous baths generally will contain over 1.7% percent carbon and may contain as much as 5% carbon and at least about 0.5% silicon and may contain as much as 6% silicon.
U.S. Pat. No. 3,728,107 to Loricchio discloses the addition of silicon carbide pelleted with chromite to molten iron for homogenizing the microstructure to control the hardness. The patent also discloses that, in general, the invention relates to cast iron which is understood to include any carbon iron alloy containing more than 1.7% total carbon and, more particularly, up to about 4% carbon. Such alloys may contain from 0.5 to 3.0% silicon and from 0.5 to 1.0% manganese.
U.S. Pat. No. 3,998,664 to Rote discloses a heat treated cast iron wherein the carbon and silicon contents are controlled to produce a white iron as cast in a sand mold and the sulfur content is in excess of that required to combine with all the manganese in the iron. The iron is annealed to produce temper carbon and a ferrous matrix containing a uniform distribution of iron sulfide particles of finite size.
U.S. Pat. No. 4,072,511 to Coyle discloses a method for producing cast iron including the steps of providing an initial cupola charge having a silicon content less than the silicon content required, melting the charge, conducting the melt to a mixing vessel, substantially increasing the silicon content of the melt by adding granular silicon carbide to the mixing vessel while simultaneously agitating the melt to achieve a good mix and conducting the silicon-enriched melt to a holding vessel or a molding line.
U.S. Pat. No. 4,096,002 to Ikawa, et al. discloses high duty ductile cast iron with super plasticity containing some carbide stabilizing elements, such as, manganese or molybdenum, to have the maximum strength rate sensitivity factor of more than 0.3 and having a very refined grain matrix structure.
U.S. Pat. No. 4,222,793 to Grindahl discloses a method for making high stress nodular iron gears which includes: casting nodular iron blank; heating blank to ferritize its microstructure prior to cutting teeth into the blank; heating it in a non-oxidizing environment to an austenitic phase dissolved-carbon-content of about 0.7% to about 1.1%; rapidly quenching the austentized casting to an acicular-bainite-forming isothermal transformation temperature; isothermally transforming the austenite at that temperature to at least 50% acicular-bainite before cooling; and shot peening at least the roots of the teeth to impart the residual compressive stresses thereto.
U.S. Pat. No. 4,396,442 to Nakamura, et al. discloses a ductile cast iron roll which comprises 3.0 to 3.8% C, 1.5 to 2.5% Si, 0.2 to 1.0% Mn, 0.01 to 0.2% P, less than 0.06% S, 0.7 to 3.0% Ni, 0.1 to 0.6% Cr, 0.1 to 0.8% Mo, 0.02 to 0.1% Mg, balance iron and unavoidable impurities and the base structure having a fine two-phase structure of ferrite mingled with pearlite.
U.S. Pat. No. 4,435,226 to Neuhauser, et al. discloses a wear resistant cast iron alloy having a tempered structure with spheroidal graphite separation comprised of 1.5 to 3.0% carbon, 3.0 to 6.0% silicon, 0.1 to 2.0% manganese, along with other elements.
U.S. Pat. No. 4,475,956 to Kovacs, et al. discloses a method of making high strength ferritic ductile iron parts in which the iron alloy melt consists essentially of by weight 3.9 to 6.0% silicon, 3.0 to 3.5% carbon, 0.1 to 0.3% manganese, 0 to 0.35% molybdenum, at least 1.25% nickel, no greater than 0.015% sulfur and 0.6% phosphorus, the remainder iron, the melt having been subjected to a nodular agent to form graphite nodules upon solidification.
U.S. Pat. No. 4,484,953 to Kovacs, et al. discloses a method of making ductile cast iron with improved strength having a matrix of acicular ferrite and bainite. The cast iron melt by weight consists of 3.0 to 3.6% carbon, 3.5 to 5% silicon, 0.7 to 5% nickel, 0 to 0.3% molybdenum, greater than 0.015% sulfur, greater than 0.06% phosphorus, and the remainder being iron, the melt being subjected to a nodularizing agent and solidified.
U.S. Pat. No. 4,596,606 to Kovacs, et al. discloses a method of making compacted graphite cast iron wherein a ferrous alloy is melted consisting essentially of, by weight, 3 to 4% carbon, 2 to 3% silicon, 0.2 to 0.7% manganese, 0.25 to 0.4% molybdenum, 0.5 to 3.0% nickel, up to 0.002% sulfur, up to 0.02% phosphorus and impurities or contaminants up to 1.0%, with the remainder being essentially iron. The melt is subjected to a graphite modifying agent to form compacted graphite upon solidification.
U.S. Pat. No. 4,619,713 to Fuenaga discloses a method for producing nodular graphite cast iron comprising pouring a melt having a nodular graphite cast iron composition into a mold; solidifying the melt in the mold to form a casting; removing the casting from the mold at a predetermined temperature above the A.sub.1 transformation temperature; rapidly cooling the casting at a cooling rate sufficient to prevent the generation of pearlite; stopping the rapid cooling at a temperature above the M.sub.s ; substantially isothermally transforming the casting to form a matrix structure consisting essentially of bainite; and cooling the casting to normal temperature.
U.S. Pat. No. 4,666,533 to Kovacs, et al. discloses a hardenable cast iron and the method of making the cast iron, wherein the cast iron melt has by weight percent a carbon equivalent equal to 4.3 to 5.0 percent, 0.55 to 1.2% manganese, 0.5 to 3.0% nickel, and the remainder being essentially iron.
U.S. Pat. No. 4,737,199 to Kovacs discloses a machinable ductile or semiductile cast iron and method for making the same which begins by forming a ferrous alloy melt consisting essentially by weight, of 3 to 4% carbon, 2.0 to 3.0% silicon, 0.1 to 0.9% manganese, up to 0.02% phosphorus, up to 0.002% sulfur, up to 1% contaminants or impurities, 0 to 0.4% molybdenum, 0 to 3.0% nickel or copper, and the remainder being substantially iron.
In addition to the patents describing cast iron compositions, U.S. Pat. No. 3,951,697 to Sherby, et al. discloses a method for treating ultra high carbon steel including heat treatment and mechanical working under sufficient deformation to refine the iron grade and spheroidize the cementite. An alternative method is disclosed which includes mixing and sintering fine cementite containing-iron alloy powders and iron powders.
All of the above-mentioned prior art attempts to prepare a cast iron having the desired properties have met with limited success. Uniformity of the physical characteristics throughout the bulk of the material has not been achieved to the degree which is desirable.
Therefore, it is a primary object of the present invention to prepare a cast iron having uniform structure and physical properties and improved mechanical properties, such as, thermal and stress stability with little or no transformation to martensite, good elongation, and good ductility, resulting in a strong, steel-like highly machinable material.
It is another object of the present invention to provide a method for preparing a strong cast iron with uniform solute distribution (i.e., manganese being evenly distributed in the matrix) for uniform reaction during heat treatment and for uniform properties.
Furthermore, it is an object of the present invention to prepare a cast iron with lower than typical graphite levels. It is yet another object of the invention to prepare a cast iron with a wide margin for heat treatment operations and with an easy control of the matrix structure.